Recent trends in condensers for automobiles, which receive a gaseous refrigerant, condense the refrigerant through heat exchange with air, and then discharge to an evaporator via an expansion means, have produced compact designs with high performance heat exchange characteristics, in accordance with the demand of small size and lightweight construction desired for car-related parts. A typical parallel flow type condenser includes a plurality of flat tubes with a plurality of corrugated fin, each corrugated fin being intervened between adjacent flat tubes, and a pair of headers to which each flat tube is connected at both ends thereof.
For aid in understanding, the reader is referred to FIG. 13. A parallel flow type condenser 60 includes first and second headers 61 and 62, a plurality of flat tubes 63, and a plurality of corrugated fins 64 disposed between adjacent flat tubes 63. Both ends of each flat tube are connected between the first and second headers 61 and 62, and at least one baffle 65 is provided within each header 61, 62 so that the refrigerant in the condenser makes multiple passes with each defined by flat tubes 63. Thus, refrigerant flows through the condenser in a zigzag pattern. The condenser with the above construction is smaller in size, more lightweight, and yet of high efficiency in heat transfer than a conventional serpentine type condenser. Therefore, the parallel flow type condenser is widely employed in automobile air conditioning systems.
In general, the refrigerant is introduced into a condenser in a vapor phase, and as the refrigerant flows from an inlet toward an outlet the refrigerant is completely changed into a liquid phase in the area on the outlet side after experiencing a gas/liquid two-phase state. Accordingly, the refrigerant exits the condenser in liquid phase to an external element of a refrigerant circuit. Namely, a vapor-abundant phase of the refrigerant flows through an upper area of the condenser shown in FIG. 13, while a liquid-abundant phase condensed from the vapor phase gradually increases approaching an lower area of the condenser, and therefore, it appears that the two-phase refrigerant flows through the condenser as a whole. During the phase change of the refrigerant, a thin liquid film, which is formed on the inside wall of each flat tube positioned in the area through which the vapor-adundant phase flows, acts as a thermal resistance hindering heat transfer between the refrigerant and the air. Furthermore, due to the rapid flow rate of the vapor phase as compared to the liquid phase, the liquid film acts as a flow resistance to the flow of the refrigerant through the condenser so that a pressure drop, i.e. pressure loss, takes place between the inlet and the outlet, which necessarily increases system energy requirements.
Commonly, it is important in designing a condenser to provide an increased heat transfer area and yet a lower pressure drop on the refrigerant side in order to enhance the performance of the condenser. Methods of increasing the heat transfer area of the flat tubes include two alternatives: one is to decrease the hydraulic diameter of each inside flow path which are formed within each flat tube to allow the refrigerant to be passed therethrough, while the other is to increase the number of passes so as to make the length of the overall fluid paths for the refrigerant passage longer, each pass including a plurality of flat tubes.
As for decreasing the hydraulic diameter of inside flow paths, U.S. Pat. No. 4,998,580 discloses a tube having a plurality of fluid flow paths formed-by a undulating spacer within the tube. Each of the fluid flow paths has a very small hydraulic diameter. However, the hydraulic diameters of the fluid flow paths are so small that a higher pressure drop developer in each pass due to the corresponding increse of refrigerant passage resistance. In a condenser to which the tubes each having such a small size of fluid flow paths are utilized, the overall length of fluid paths for the refrigerant passage must be shorter than a condenser with relatively large hydraulic diameter tubes or more passes, in order to account for the higher pressure drop in each pass. Accordingly, in U.S. Pat. No. 4,998,580, if the number of the refrigerant passes increases, for example, over three, too much pressure drop on the refrigerant side occurs and results in an increase in of system energy requirements.
As for the method of increasing the overall fluid paths for the refrigerant passage, as shown in FIG. 13, a plurality of baffles or partitions are provided in the headers, the provision of which causes the refrigerant introduced into the condenser to flow across the condenser in a zigzag fashion, and as a consequence, increasing the effective cross-sectional area of tubes. It seems that this design is more frequently used in automobile air conditioning systems. In this condenser design, considering the phase change of the refrigerant from vapor into liquid occurring during passage of the refrigerant through the condenser, effective heat transfer area or the number of tubes in the uppermost pass on the inlet side is relatively larger and effective heat transfer area of passes are progressively reduced toward the lowermost pass on the outlet side because of large volume and rapid flow rate of the gaseous refrigerant as compared with the liquid refrigerant. Due to these considerations, most heat exchange takes place in the uppermost pass on the inlet side and, in addition, the flow resistance of refrigerant across the condenser is reduced as well.
However, when tubes having an excessively small hydraulic diameter tubes or overly long fluid paths are selected to enhance the heat transfer efficiency of condensers, the heat transfer effiency does increase but the load exerted on a compressor rises according to the increase of pressure drop due to large flow resistance of the refrigerant between the inlet and the outlet of the condenser. Accordingly, to prevent an excessive pressure drop from taking place and to obtain the desired heat transfer efficiency, it is required that the number of U-turns in flow of the refrigerant be minimized for the condenser tubes with a small size of hydraulic diameters on one hand and the number of U-turns be at least two for the condenser with tubes of relatively large hydraulic diameters on the other hand.
In the meantime, for a condenser in which the length of fluid paths of the refrigerant is established long by allowing the refrigerant to flow in a zigzag fashion because of provision of at least one baffle in the headers, prior art is known that includes by-pass passageway formed at the center of the baffles to make a pressure drop according to increase of the fluid path length to be minimized and to permit a liquid refrigerant condensed passing through passes to be by-passed to an outlet side of the condenser.
For example, U.S. Pat. No. 4,243,094 (the "'094" Patent) discloses a condenser including a pair of headers, a plurality of tubes (conduit members) with fins surrounding each of the tubes, and baffles having a bore. Bores are of a size which allow the condensed liquid through each pass to flow therethrough by capillary action into a adjacent lower chamber in the same header without passing through a subsequent pass. The '094 Patent describes that centrally disposed bores are so small that they act as capillary tubes and effectively prevent gaseous fluid from passing therethrough. Therefore, the bores insure that only fluid in a liquid state will pass therethrough.
However, since the '094 Patent does not mention expressly the number of passes for the refrigerant passage, the sizes of the hydraulic diameters of tubes and the bores (by-pass passageways), and the relation therebetween, it is difficult to apply the '094 Patent to the actual design of a condenser. For example, what heat transfer efficiency would be obtained based on the number of passes selected for the refrigerant passage? How are the sizes of by-pass passageways defined? And how should the by-pass passageways be established in view of the number of passes for the refrigerant passage and the hydraulic diameter of tubes? Furthermore, it is difficult to form bores in the baffles and to dispose the baffles within the headers, considering that the bores should have a small diameter and a long length to accomplish capillary action in fluid flow.
Another prior art document concerning the by-pass of the condensed liquid refrigerant is Japanese Unexamined Utility Model No. 63-173688 (application No. 62-064734) which discloses, as shown in FIGS. 14 and 15a and 15b, a condenser including a pair of headers 70 having tubes 78 each connected to the headers at both ends thereof, and a baffle means 73 having an upper member 74, a meshed member 77 and a lower member 75. The baffle means 73 divides an internal space of each header 70 into upper and lower chambers 71 and 72, respectively. Each upper and lower member 74 and 75 is provided with a hole 76, and liquid refrigerant is by-passed from the upper chamber 71 into the lower chamber 72 through the holes 76 and the meshed member 77. However, the condenser with the above construction does not disclose the relation between the heat transfer efficiency and the pressure drop, the number of passes for the refrigerant passage, the size of by-pass passageways, and the relation therebetween, except simple description about by-passing the liquid refrigerant through the by-pass passageways formed in the baffle means.